Inner hair cell stimulation model for use by a cochlear implant system

ABSTRACT

The stimulation provided in the electrically stimulated cochlea is modulated in accordance with the amplitude of a received acoustic signal and the onset of a sound in a received acoustic signal to provide increased sound perception. An onset time that corresponds to the onset of a sound is detected in an acoustic signal associated with a frequency band. A forcing voltage and a transmitting factor are determined, wherein the forcing voltage and the transmitting factor are associated with the frequency band at the detected onset time. The acoustic signal is modulated as a function of the forcing voltage and the transmitting factor to generate an output signal. The generated output signal can be used to stimulate the cochlea. The modulation strategy can be used in conjunction with sound processing strategies that employ frequency modulation, amplitude modulation, or a combination of frequency and amplitude modulation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/085,925, filed Nov. 21, 2013 (now U.S. Pat. No. 9,254,384), which iscontinuation of U.S. patent application Ser. No. 12/414,262, filed Mar.30, 2009 (now U.S. Pat. No. 8,615,302), which is a continuation of U.S.patent application Ser. No. 10/992,390, filed Nov. 17, 2004 (now U.S.Pat. No. 7,522,961), Priority is claimed to all these applications, andall are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present disclosure relates to implantable neurostimulator devicesand systems, for example, cochlear stimulation systems, and to soundprocessing strategies employed in conjunction with such systems.

BACKGROUND

Prior to the past several decades, scientists generally believed that itwas impossible to restore hearing to the profoundly deaf. However,scientists have had increasing success in restoring normal hearing tothe deaf through electrical stimulation of the auditory nerve. Theinitial attempts to restore hearing were not very successful, aspatients were unable to understand speech. However, as scientistsdeveloped different techniques for delivering electrical stimuli to theauditory nerve, the auditory sensations elicited by electricalstimulation gradually came closer to sounding more like normal speech.

The electrical stimulation is implemented through a prosthetic device,known as a cochlear implant, which is implanted in the inner ear torestore partial hearing to profoundly deaf patients. Such cochlearimplants generally employ an electrode array that is inserted into thecochlear duct. One or more electrodes of the array selectively stimulatedifferent auditory nerves at different places in the cochlea based onthe pitch of a received sound signal.

Within the cochlea, there are two main cues that convey “pitch”(frequency) information to the patient. There are (1) the place orlocation of stimulation along the length of a cochlear duct and (2) thetemporal structure of the stimulating waveform. Sound frequencies aremapped to a “place” in the cochlea, generally from low to high soundfrequencies mapped from the apical to basilar direction. The electrodearray is fitted to the patient to arrive at a mapping scheme such thatelectrodes near the base of the cochlea are stimulated with highfrequency signals, while electrodes near the apex are stimulated withlow frequency signals. The stimulation signals provided to theelectrodes model the received acoustic signal associated with aparticular frequency band. However, the stimulation signals do notnecessarily accurately represent the signals generated by nerves.

FIG. 1 presents a cochlear stimulation system 10 that includes a soundprocessor portion 12 and a cochlear stimulation portion 20. The soundprocessor portion 12 includes a microphone 14 and a sound processor 18.The microphone 14 can be connected directly to the sound processor 18.Alternatively, the microphone 14 can be coupled to the sound processor18 through an appropriate communication link 16. The cochlearstimulation portion 20 includes an implantable cochlear stimulator 22and an electrode array 24. The electrode array 24 is adapted to beinserted within the cochlea of a patient. The electrode array 24includes a plurality of electrodes (not shown) that are distributedalong the length of the array and are selectively connected to theimplantable cochlear stimulator 22.

The electrode array 24 may be substantially as shown and described inU.S. Pat. No. 4,819,647 or U.S. Pat. No. 6,129,753, both patentsincorporated herein by reference. Electronic circuitry within theimplantable cochlear stimulator 22 allows a specified stimulationcurrent to be applied to selected pairs or groups of the electrodes (notshown) included within the electrode array 24 in accordance with aspecified stimulation pattern defined by the sound processor 18.

The sound processor 18 and the implantable cochlear stimulator 22 areelectronically coupled through a suitable communication link 26. In animplementation, the microphone 14 and the sound processor 18 comprise anexternal portion of the cochlear stimulation system 10, and theimplantable cochlear stimulator 22 and the electrode array 24 comprisean internal, or implanted, portion of the cochlear stimulation system10. Thus, the communication link 26 is a transcutaneous (through theskin) link that allows power and control signals to be sent from thesound processor 18 to the implantable cochlear stimulator 22.

In another implementation, the implantable cochlear stimulator 22 cansend information, such as data and status signals, to the soundprocessor 18 over the communication link 26. In order to facilitatebidirectional communication between the sound processor 18 and theimplantable cochlear stimulator 22, the communication link 26 caninclude more than one channel. Additionally, interference can be reducedby transmitting information on a first channel using anamplitude-modulated carrier and transmitting information on a secondchannel using a frequency-modulated carrier.

In an implementation in which the implantable cochlear stimulator 22 andthe electrode array 24 are implanted within the patient, and themicrophone 14 and the sound processor 18 are carried externally (notimplanted) by the patient, the communication link 26 can be realizedthough use of an antenna coil in the implantable cochlear stimulator 22and an external antenna coil coupled to the sound processor 18. Theexternal antenna coil can be positioned so that it is aligned with theimplantable cochlear stimulator 22, allowing the coils to be inductivelycoupled to each other and thereby permitting power and information,e.g., a stimulation signal, to be transmitted from the sound processor18 to the implantable cochlear stimulator 22. In another implementation,the sound processor 18 and the implantable cochlear stimulator 22 canboth be implanted within the patient, and the communication link 26 canbe a direct-wired connection or other suitable link as shown in U.S.Pat. No. 6,308,101, incorporated herein by reference.

In the cochlear stimulation system 10, the microphone 14 senses acousticsignals and converts the sensed acoustic signals to correspondingelectrical signals. The electrical signals are sent to the soundprocessor 18 over an appropriate communication link 16, such as acircuit or bus. The sound processor 18 processes the electrical signalsin accordance with a sound processing strategy and generates controlsignals used to control the implantable cochlear stimulator 22. Suchcontrol signals can specify or define the polarity, magnitude, location(which electrode pair or group is intended to receive the stimulationcurrent), and timing (when the stimulation current is to be applied tothe intended electrode pair or group) of the stimulation signal, such asa stimulation current, that is generated by the implantable cochlearstimulator 22.

It is common in the cochlear stimulator art to condition the magnitudeand polarity of the stimulation current applied to the implantedelectrodes of the electrode array 24 in accordance with a specifiedsound processing strategy. A sound processing strategy involves defininga pattern of stimulation waveforms that are applied as controlledelectrical currents to the electrodes of an electrode array 24 implantedin a patient. Stimulation strategies can be implemented by modulatingthe amplitude of the stimulation signal or by modulating the frequencyof the stimulation signal.

SUMMARY

The present inventors recognized the need to generate stimulationsignals that account for the neural response of a healthy ear toreceived acoustic signals. Accordingly, the methods and apparatusdescribed here implement techniques for enhancing sound as perceivedthrough a cochlear implant. More specifically, the methods and apparatusdescribed here implement techniques for using the inner hair cell modelto emphasize the onsets of sound signals as perceived through a cochlearimplant.

In general, in one aspect, the techniques can be implemented to includedetecting an onset time corresponding to an onset of a sound in anacoustic signal associated with a frequency band; determining a forcingvoltage and a transmitting factor, wherein the forcing voltage and thetransmitting factor are associated with the frequency band at thedetected onset time; and modulating the acoustic signal as a function ofthe forcing voltage and the transmitting factor to generate an outputsignal.

The techniques also can be implemented such that modulating the acousticsignal further comprises modulating the frequency of the acousticsignal. The techniques further can be implemented to include generatingthe output signal in accordance with a Frequency Modulated Stimulation(FMS) strategy such as that described in U.S. published patentapplication 2007/0239227, incorporated herein by reference.Additionally, the techniques can be implemented such that modulating theacoustic signal further comprises modulating the amplitude of theacoustic signal. Further, the techniques can be implemented to includegenerating the output signal in accordance with one of a ContinuousInterleaved Stimulation (CIS) strategy, a Simultaneous AnalogStimulation (SAS) strategy, or a Hi-Resolution (HiRes) strategy.

The techniques also can be implemented such that modulating the acousticsignal further comprises modulating the frequency of the acoustic signaland the amplitude of the acoustic signal. Further, the techniques can beimplemented such that modulating the acoustic signal further comprisesmodulating the acoustic signal as a function of a modulation constant.The techniques also can be implemented such that the generated outputsignal decays at a variable rate. The techniques further can beimplemented such that the generated output signal comprises an acousticsignal. Additionally, the techniques can be implemented to includemapping the generated output signal to an electrical signal and applyingthe electrical signal to one or more electrode pairs of a cochlearimplant.

In general, in another aspect, the techniques can be implemented toinclude a detector configured to detect an onset time corresponding toan onset of a sound in an acoustic signal associated with a frequencyband and circuitry configured to determine a forcing voltage and atransmitting factor, wherein the forcing voltage and the transmittingfactor are associated with the frequency band at the detected onsettime, and to modulate the acoustic signal as a function of the forcingvoltage and the transmitting factor to generate an output signal.

The techniques also can be implemented to include circuitry configuredto modulate the frequency of the acoustic signal. Further, thetechniques can be implemented to include circuitry configured togenerate the output signal in accordance with a Frequency ModulatedStimulation strategy. Additionally, the techniques can be implemented toinclude circuitry configured to modulate the amplitude of the acousticsignal. Further still, the techniques can be implemented to includecircuitry configured to generate an output signal in accordance with oneof a Continuous Interleaved Stimulation strategy, a Simultaneous AnalogStimulation strategy, or a HiRes strategy.

The techniques also can be implemented to include circuitry configuredto modulate the frequency of the acoustic signal and the amplitude ofthe acoustic signal. Further, the techniques can be implemented toinclude circuitry configured to modulate the acoustic signal as afunction of a modulation constant. Additionally, the techniques can beimplemented such that the generated output signal decays at a variablerate. The techniques further can be implemented such that the generatedoutput signal comprises an acoustic signal. The techniques also can beimplemented to include circuitry configured to map the generated outputsignal to an electrical signal and apply the electrical signal to one ormore electrode pairs of a cochlear implant.

In general, in another aspect, the techniques can be implemented toinclude detecting a time corresponding to a sound in an acoustic signal;modeling a neural response to the sound in the acoustic signal at thedetected time; and generating an output signal by modulating theacoustic signal as a function of the modeled neural response.

The techniques also can be implemented such that modeling a neuralresponse comprises determining a forcing voltage as a function of anamplitude associated with the acoustic signal at the detected time anddetermining a transmitting factor as a function of a quantity ofneurotransmitter available at the detected time. Additionally, thetechniques can be implemented such that modulating the acoustic signalcomprises modulating one or more of the frequency of the acoustic signaland the amplitude of the acoustic signal. Further, the techniques can beimplemented such that modulating the acoustic signal comprisesmodulating the acoustic signal as a function of a modulation constant.

The techniques also can be implemented such that the generated outputsignal decays at a variable rate. Additionally, the techniques can beimplemented such that the generated output signal comprises an acousticsignal. Further, the techniques can be implemented to include mappingthe generated output signal to an electrical signal and applying theelectrical signal to one or more electrode pairs of a cochlear implant.

The techniques described in this specification can be implemented torealize one or more of the following advantages. For example, thetechniques can be implemented to emphasize the onsets of sound signalsto account for the neural response of a healthy ear and thus improvesound clarity and speech recognition, especially under difficultlistening conditions. The techniques also can be implemented to decreasethe power consumption of a cochlear implant system implementing a soundprocessing strategy.

These general and specific aspects can be implemented using anapparatus, a method, a system, or any combination of an apparatus,methods, and systems. The details of one or more implementations are setforth in the accompanying drawings and the description below. Furtherfeatures, aspects, and advantages will become apparent from thedescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a cochlear stimulation system.

FIG. 2 is a functional block diagram of a sound processing system.

FIG. 3 presents exemplary frequency maps that can be used in conjunctionwith a sound processing strategy.

FIG. 4 presents a model of an inner hair cell.

FIG. 5 presents a simplified model of an inner hair cell.

FIG. 6 presents a graphical depiction of neural response to a pure tone.

FIG. 7 is a flowchart of a method of stimulating a cochlea.

Like reference symbols indicate like elements throughout thespecification and drawings.

DETAILED DESCRIPTION

FIG. 2 presents a functional block diagram of a system arranged toimplement a sound processing strategy. Such sound processing strategycan be implemented using any combination of circuitry and programmedinstructions, including one or more of a programmable logic device, afield-programmable gate array, an application-specific integratedcircuit, and a general purpose processor executing programmedinstructions.

In the sound processing system 100, an audio signal 102 is provided asan input to a bank of bandpass filters 104, which separates the audiosignal 102 into individual frequency bands or channels. For example, ifthe audio signal 102 is provided to a bank of K bandpass filters, thenthe audio signal 102 is separated into K individual frequency bands. Inanother implementation, different types and combinations of filters canbe employed to separate the audio signal 102 into individual frequencybands, such as notch filters, high-pass filters, and low-pass filters.

As the audio signal 102 is provided to the bank of bandpass filters 104,individual bandpass filters output filtered signals. For example, thebank of bandpass filters 104 includes a bandpass filter 106corresponding to a first frequency band, a bandpass filter 108corresponding to a second frequency band, and a bandpass filter 110corresponding to a k_(th) frequency band. As the audio signal 102 isprovided to the bank of bandpass filters 104, the bandpass filter 106corresponding to the first frequency band outputs a filtered signal 112associated with the first frequency band, the bandpass filter 108corresponding to the second frequency band outputs a filtered signal 113associated with the second frequency band, and the bandpass filter 110corresponding to the k_(th) frequency band outputs a filtered signal 114associated with the k_(th) frequency band. Thus, each filtered signal isassociated with a frequency band that represents a subset of the audiosignal 102.

A sound processing strategy can be developed, which employs a pluralityof specific frequency bands. The plurality of specific frequency bandscorresponding to the particular sound processing strategy can be definedusing a frequency map. FIG. 3 presents an example of a frequency mapassociated with the SAS strategy 310 and a frequency map associated withthe CIS strategy 320. Other sound processing strategies, such as the FMSstrategy, can be implemented using either of the frequency mapspresented in FIG. 3, or by using alternative frequency maps. After thefrequency map corresponding to a particular sound processing strategyhas been defined, the bank of bandpass filters 104 can be configured toseparate the audio signal 102 into frequency bands that correspond tothe defined frequency map.

In another implementation, the audio signal 102 may undergo otherprocessing before being provided as input to the bank of bandpassfilters 104. For example, the audio signal 102 may originate as acousticinformation sensed by a microphone, which is then converted into anelectrical signal representing an audio signal. The electrical signalcan further be converted to a digital signal in an analog-to-digitalconverter, and then be subjected to automatic gain control (AGC)processing using an AGC algorithm. The AGC algorithm serves to compressthe dynamic range of the audio signals to provide a more consistentlevel of stimulus to the electrodes and to equalize the level betweensound sources that are removed from the listener by differing distances.

The signals output from the bank of bandpass filters 104 are thenprovided to a bank of inner hair cell model (IHCM) processors 116. Thefiring rate of auditory nerve fibers increases with the amplitude of areceived acoustic signal and is highest near the onset of a sound.Therefore, the IHCM processors included in the bank of IHCM processors116 are configured to model the firing rate of auditory nerve fibers ina healthy ear. The bank of IHCM processors 116 includes an IHCMprocessor 118 corresponding to the first frequency band, an IHCMprocessor 120 corresponding to the second frequency band, and an IHCMprocessor 122 corresponding to the k_(th) frequency band. The IHCMprocessors of the bank of IHCM processors 116 receive as input thefiltered signals output from the corresponding bandpass filters in thebank of bandpass filters 104. For example, the IHCM processor 118corresponding to the first frequency band receives as input the filteredsignal 112 associated with the first frequency band from the bandpassfilter 106 corresponding to the first frequency band. Similarly, theIHCM processors 120 and 122 corresponding to the second frequency bandand the k_(th) frequency band receive as input the filtered signals 113and 114 associated with the second frequency band and the k_(th)frequency band respectively.

The IHCM processors of the bank of IHCM processors 116 then transducethe received filtered signals to produce processed signals.Additionally, the IHCM processors determine a modulation factor X_(o),as described below, and apply the modulation factor X_(o) to theprocessed signals to produce modulated signals. For example, the IHCMprocessor 118 corresponding to the first frequency band transduces thereceived filtered signal 112 associated with the first frequency band toproduce a processed signal. The IHCM processor 118 also determines amodulation factor X_(o) associated with the first frequency band andapplies the modulation factor X_(o) to the processed signal to produce amodulated signal M₁ 124 associated with the first frequency band.

In an implementation, each IHCM processor of the bank of IHCM processors116 includes an envelope detector, which receives as input the filteredsignal output by the corresponding bandpass filter. The envelopedetector included with the IHCM processor is configured to determine anenvelope of the received filtered signal and to produce a processedsignal, such as a representative envelope signal. For example, thefiltered signal 112 associated with the first frequency band is providedas input to the envelope detector included with the IHCM processor 118corresponding to the first frequency band, which determines the envelopeof the filtered signal 112 and produces a processed signal. The IHCMprocessor 118 then produces a modulated signal M₁ 124 associated withthe first frequency band using the processed signal and the modulationfactor X_(o). Each envelope detector can also include a rectifier, suchas a half-wave rectifier or a full-wave rectifier, that rectifies thefiltered signal output from the corresponding bandpass filter of thebank of bandpass filters 104 before the envelope of the filtered signalis determined.

Additionally, the envelope detectors included with the IHCM processorsof the bank of IHCM processors 116 can comprise integrators thatdetermine an average amplitude of a signal for a given interval. Forexample, upon receiving the filtered signal 112 associated with thefirst frequency band, the envelope detector included with the IHCMprocessor 118 corresponding to the first frequency band determines theenvelope of the filtered signal 112 for an interval, such as a frame. Atthe end of the interval, the envelope detector produces a processedsignal that represents the average amplitude of the filtered signal 112associated with the first frequency band for that interval. As describedabove, the IHCM processor 118 corresponding to the first frequency bandthen produces a modulated signal M₁ 124 associated with the firstfrequency band using the processed signal and the modulation factorX_(o). Each envelope detector can be further configured to set theaverage amplitude value of a received filtered signal to an initialstate prior to the start of a new interval.

In another implementation, each IHCM processor of the bank of IHCMprocessors 116 includes a phase locking mechanism (PLM). In a healthycochlea, the spiral ganglion cells located at the lower frequency end ofthe basilar membrane respond in phase with the vibration period of thebasilar membrane. Similarly, the spiral ganglion cells located at thehigher frequency end of the basilar membrane respond in phase at integermultiples of the vibration period of the basilar membrane. This responsebehavior is emulated using the PLM.

Each PLM generates pulse signals at integer multiples of the periods ofthe received filtered signal associated with the corresponding frequencyband. The PLM processor determines the periods by monitoring theintervals between the positive-to-negative zero crossings of thereceived filtered signal. Each PLM processor can modulate the number ofperiods that are skipped between two consecutive pulses by the amplitudeof the filtered signal associated with the frequency band. The number ofperiods N_(i) to be skipped for a frequency band i having a centerfrequency Fc can be calculated using the linear approximation shown inequation 1.

$\begin{matrix}{N_{i} = {{{ceil}( \frac{M_{i}}{1/{Fc}_{i}} )} = {{ceil}( {M_{i}*F\; c_{i}} )}}} & (1)\end{matrix}$

As shown in equation 2, the parameter M_(i) can be modulated betweenM_(min) and M_(max) milliseconds (ms) by the amplitude of the signaloutput from the bandpass filter associated with the frequency band i,BPF_(i). In a healthy ear, the interval between spikes typically fallsbetween M_(min)=1 and M_(max)=3 ms. The parameter BPF_(i,max) representsthe maximum output of the bandpass filter when the input is calibratedto a particular value, such as 65 decibel sound pressure level whitenoise.

$\begin{matrix}{M_{i} = {{\frac{M_{\max} - M_{\min}}{{BPF}_{i,\max}}*{BPF}_{i}} + M_{\min}}} & (2)\end{matrix}$

The modulated signals output from the IHCM processors of the bank ofIHCM processors 116 are converted to electrical signals usingacoustic-to-electrical mappings associated with the correspondingfrequency bands. Each of the resulting electrical signals is thenapplied to electrodes of a cochlear implant to provide a stimulationsignal. For example, the modulated signal M₁ 124 associated with theIHCM processor 118 corresponding to the first frequency band isconverted from an acoustic signal to an electrical signal using theacoustic-to-electrical mapping 132 associated with the first frequencyband. Similarly, the modulated signals M₂ 134 and M_(k) 144 areconverted to electrical signals using the acoustic-to-electricalmappings 142 and 152 associated with the second frequency band and thek_(th) frequency band respectively. The electrical signals output fromthe acoustic-to-electrical mappings are then provided as stimulationsignals to the electrodes of a cochlear implant.

In another implementation, a signal generation queue can be employed toensure that two or more stimulation signals are never delivered to theelectrodes of a cochlear implant simultaneously. For example, the signalgeneration queue can monitor the modulated signals output from the bankof IHCM processors 116, such as the modulated signal M₁ 124 associatedwith the first frequency band and the modulated signal M_(k) 144associated with the k_(th) frequency band. The signal generation queuecan then control the output of stimulation signals associated with themodulated signals M₁ 124 and M_(k) 144 in a temporally-separated,first-in-first-out manner. By preventing the simultaneous delivery oftwo or more stimulation signals, the unfavorable effects of electricalchannel interaction can be minimized.

FIG. 4 presents a model of an inner hair cell. A plurality of inner haircells are arranged within the organ of Corti, which lies on the basilarmembrane inside of the cochlea. Acoustic signals entering the cochleacause the basilar membrane to vibrate, or deflect from a neutralposition. In turn, the vibration of the basilar membrane createsmechanical stimulus that triggers one or more of the inner hair cells.The inner hair cells transduce the mechanical stimulus associated withthe vibration of the basilar membrane into neural impulses, which aretransmitted along the auditory nerve.

As demonstrated by the inner hair cell model 400, an individual innerhair cell 410 includes a plurality of hairs 420 that receive mechanicalstimulus associated with the vibration of the basilar membrane. When oneor more of the plurality of hairs 420 included in the inner hair cellare triggered by a received mechanical stimulus, the inner hair cell 410releases neurotransmitter into the synaptic cleft 450 that lies betweenthe inner hair cell 410 and one or more adjoining nerve fibers 460, 480,and 490. Each of the adjoining nerve fibers 460, 480, and 490 isassociated with a nerve cell. The inner hair cell 410 can communicatewith a plurality of nerve fibers 460, 480, and 490. However, each of thenerve fibers 460, 480, and 490 can only be triggered by an individualinner hair cell 410.

The inner hair cell 410 also includes a global store 430 and animmediate store 440. The global store 430 functions as a factory toproduce the neurotransmitter that is released by the inner hair cell 410in response to received stimulus. In the global store 430, smallquantities of neurotransmitter are packaged into individual vesicles forstorage. After production, the vesicles of neurotransmitter pass fromthe global store 430 to the immediate store 440 at a variable rate,depending on the relative concentrations of neurotransmitter in theglobal store 430 and the immediate store 440.

As described above, when one or more of the plurality of hairs 420 ofthe inner hair cell 410 are triggered by mechanical stimulus, the innerhair cell 410 releases the neurotransmitter stored in a number ofvesicles in the immediate store 440 into the synaptic cleft 450. Thenumber of vesicles in the immediate store 440 that may be used by theinner hair cell 410 to release neurotransmitter represents atransmitting factor, which influences whether an impulse can betriggered. As neurotransmitter is released into the synaptic cleft 450,the concentration of neurotransmitter in the proximity of one or more ofthe nerve fibers 460, 480, and 490 increases, which raises theprobability that one or more of the nerve fibers 460, 480, and 490 willbe triggered to generate an impulse 470 that will be transmitted to theauditory nerve.

When the immediate store 440 releases vesicles, the relativeconcentrations of neurotransmitter in the global store 430 and theimmediate store 440 change and vesicles of neurotransmitter aretransferred from the global store 430 to the immediate store 440.However, the rate at which the global store 430 can produce vesicles ofneurotransmitter is limited. Therefore, once the inner hair cell 410 hasreleased all of its stored vesicles, it can only continue to releasevesicles at the rate at which they are received from the global store430. The supply of vesicles in the immediate store 440 can only bereplenished once the stimulus triggering the inner hair cell 410 hasstopped. As such, the number of vesicles in the immediate store 440 atany point in time depends on several factors, including the interval oftime that has passed since the immediate store 440 last releasedneurotransmitter in response to a received stimulus and the relativeconcentration of neurotransmitter remaining in the immediate store 440after the previous release.

The number of vesicles released from the immediate store 440 dependsupon the number of vesicles in the immediate store 440 at the time themechanical stimulus is received and the perceived strength of themechanical stimulus. The perceived strength of the mechanical stimulusis associated with the amplitude of the vibration of the basilarmembrane at the location corresponding to the inner hair cell 410, whichin turn is related to the amplitude of the received acoustic signal.Therefore, an inner hair cell 410 is most likely to release a largequantity of neurotransmitter, and thereby trigger a nerve fiber 460 togenerate an impulse 470, when the amplitude of the received acousticsignal is high and the number of vesicles in the immediate store 440 ofthe inner hair cell 410 is large.

The inner hair cell model presented in FIG. 4 can be simplified as shownin FIG. 5, and the simplified model can be used to estimate the numberof vesicles of neurotransmitter in the immediate store X_(i) at aspecific point in time. The global store 510 can be modeled using thevolume of the global store G 520 and the number of vesicles in theglobal store X_(g) 530. The density of vesicles in the global store 510at a specific point in time can thus be determined by dividing thenumber of vesicles in the global store X_(g) 530 by the volume of theglobal store G 520. Similarly, the immediate store 550 can be modeledusing the volume of the immediate store I 560 and the number of vesiclesin the immediate store X_(i) 570. The density of vesicles in theimmediate store 550 at a specific point in time can thus be determinedby dividing the number of vesicles in the immediate store X_(i) 570 bythe volume of the immediate store I 560.

The number of vesicles that flow out of the immediate store 550 when astimulus is received can be modeled using the density of vesicles in theimmediate store and the voltage V forcing vesicle release 580 from theimmediate store. The voltage V forcing vesicle release 580 correspondsto force the plurality of hairs 420 of the inner hair cell 410 perceiveas a result of the vibration of the basilar membrane.

Additionally, the number of vesicles that flow from the global store 510to the immediate store 550 can be modeled according to the relativedensities of the global store 510 and the immediate store 550 inconjunction with a permeability constant 540. As vesicles are forced outof the immediate store 550 in accordance with the voltage V forcingvesicle release 580, the density of vesicles in the immediate store 550decreases. Vesicles from the global store 510 then flow to the immediatestore 550 in response to the change in the relative densities. Thepermeability constant 540 influences the rate at which the vesicles maybe transferred.

The simplified model presented with reference to FIG. 5 can be used todetermined the change in the number of vesicles in the immediate store550 with respect to time, as shown in equation 3.

$\begin{matrix}{\frac{\mathbb{d}X_{i}}{\mathbb{d}t} = {{P \cdot ( {\frac{X_{g}}{G} - \frac{X_{i}}{I}} )} - ( {V \cdot X_{i}} )}} & (3)\end{matrix}$

Using equation 3, it can be seen that the change in the number ofvesicles in the immediate store dX_(i) over time can be determined usingthe density of the vesicles in the global store X_(g)/G, thepermeability constant P, the density of the vesicles in the immediatestore X_(i)/I, the voltage V forcing vesicle release from the immediatestore, and the number of vesicles in the immediate store X_(i).

Further, the first order differential equation shown in equation 3 canbe reduced to a non-linear difference equation, as shown in equation 4.X _(i) _(n) =η−(θ+V·d _(t))X _(i) _(n-1)   (4)

In equation 4, the number of vesicles in the immediate store X at a timen can be represented as the difference between η, a constantrepresenting the density of vesicles in the global store multiplied bythe permeability constant P, and the sum of θ, a constant representingthe density of vesicles in the immediate store multiplied by thepermeability constant P, and the voltage V forcing vesicle release as afunction of time, the sum multiplied by the number of vesicles in theimmediate store X_(i) at the preceding time interval n−1.

The number of vesicles in the immediate store X_(i) at a given time n,as given by equation 4, can then be used to determine a modulationfactor X_(o). The modulation factor X_(o) can be used to model theneural response to a perceived acoustic signal associated with aparticular frequency band, and can be expressed mathematically as shownin equation 5.X _(o) =γVX _(i) _(n)   (5)

In equation 5, modulation factor X_(o) is represented as the number ofvesicles in the immediate store X_(i) at a given time n, multiplied bythe voltage V forcing vesicle release from the immediate store, and themodulation constant γ. The resulting modulation factor X_(o) can be usedto modulate the inter-pulse interval of the output of a frequencymodulated sound processing strategy, such as the FMS strategy. Byapplying the modulation factor X_(o) to the output of a frequencymodulated sound processing strategy, the stimulation signal will betterapproximate the neural response of a healthy ear to a perceived acousticsignal.

In another implementation, the modulation factor X_(o) can be used tomodulate the amplitude of the output of an amplitude modulated soundprocessing strategy, such as the CIS, SAS, or HiRes strategies. Applyingthe modulation factor X_(o) to the output of an amplitude modulatedsound processing strategy also produces a stimulation signal that betterapproximates the neural response of a healthy ear to a perceivedacoustic signal. In still another implementation, the modulation factorX_(o) can be used to modulate the output of a sound processing strategythat employs both frequency modulation and amplitude modulation.

FIG. 6 presents a graphical depiction of neural response to a pure tone.The post stimulus time excitation histogram 600 plots the discharge rate610, measured in spikes per second, over a period of time 620, measuredin seconds, of the responses of models of three nerve fibers, where eachof the nerve fibers is tuned to a different frequency band. The firstnerve fiber is tuned to a first frequency band that has a centerfrequency of 5,000 Hz, the second nerve fiber is tuned to a secondfrequency band that has a center frequency of 10,000 Hz, and the thirdnerve fiber is tuned to a third frequency band that has a centerfrequency of 20,000 Hz.

The response to the introduction of a pure tone at 5,000 Hz is depictedby the discharge rate plots associated with each of the three nervefibers. The first discharge rate plot 630 is associated with the firstnerve fiber, the second discharge rate plot 640 is associated with thesecond nerve fiber, and the third discharge rate plot 650 is associatedwith the third nerve. As shown in the post stimulus time excitationhistogram 600, discharges associated with each of the nerve fibers areseen following the introduction at time 0 of a pure tone at 5,000 Hz.

The discharge rate plots 640 and 650 associated with the second andthird nerve fibers indicate that relatively few spikes occur in responseto the introduction of the tone. However, the discharge rate plot 630associated with the first nerve fiber tuned to the frequency band havinga center frequency of 5,000 Hz indicates that a large number of spikesare generated in response to the tone. Further, the greatest number ofspikes associated with the first nerve fiber are generated within thefirst 5 ms following onset of the tone. Therefore, incorporating themodulation factor X_(o) into a sound processing strategy, as describedabove, results in a stimulation signal that corresponds more closely tothe neural response of a healthy ear.

FIG. 7 describes a method of generating a cochlear stimulation signalusing a strategy, such as the inner hair cell model, to emphasize theonsets of sound signals as perceived through a cochlear implant. In afirst step 710, an onset time that corresponds to the onset of a soundin an acoustic signal is detected. The acoustic signal is associatedwith a frequency band. In a second step 720, a forcing voltage and atransmitting factor are determined. The forcing voltage and thetransmitting factor are associated with the frequency band at thedetected onset time. Once the forcing voltage and the transmittingfactor have been determined, the third step is to modulate the acousticsignal as a function of the forcing voltage and the transmitting factorto generate an output signal.

A number of implementations have been disclosed herein. Nevertheless, itwill be understood that various modifications may be made withoutdeparting from the spirit and scope of the claims. Accordingly, otherimplementations are within the scope of the following claims.

The invention claimed is:
 1. An apparatus for generating a cochlearstimulation signal, the apparatus comprising: a plurality of bandpassfilters each for receiving an audio signal and for producing a filteredsignal comprising a center frequency; and a plurality of processors,each processor for receiving one of the filtered signals and forproducing a modulated signal, wherein each processor is configured togenerate pulses at an integer multiple of a period of the one of thefiltered signals, and wherein the integer multiple comprises: a functionof the center frequency; an amplitude of the filtered signal; and a dataindicative of a firing rate of auditory nerve fibers in a healthy ear.2. The apparatus of claim 1, further comprising mapping circuitry forconverting each modulated signal to an electrical signal.
 3. Theapparatus of claim 2, wherein the electrical signals are provided asstimulation signals to electrodes of a cochlear implant.
 4. Theapparatus of claim 3, wherein the electrical signal is transmittedwirelessly.
 5. The apparatus of claim 1, further comprising a microphonefor providing the audio signal.
 6. The apparatus of claim 1, whereineach processor comprises a phase locking mechanism.
 7. The apparatus ofclaim 6, wherein the phase locking mechanism emulates a phaserelationship between a ganglion cell response and a basilar membranevibration period in a healthy cochlea.
 8. The apparatus of claim 7,wherein the data indicative of a firing rate of auditory nerve fibers ina healthy ear comprises a maximum and a minimum interval.
 9. A method ofgenerating a cochlear stimulation signal, comprising: receiving an audiosignal by a bandpass filter; producing a filtered signal comprising acenter frequency by the bandpass filter; receiving the filtered signalby a processor; producing a modulated signal from the filtered signal bythe processor, comprising: generating pulses at an integer multiple of aperiod of the filtered signal, the integer multiple comprising: afunction of the center frequency; an amplitude of the filtered signal;and a data indicative of a firing rate of auditory nerve fibers in ahealth ear.
 10. The method of claim 9, further comprising: convertingthe modulated signal into an electrical signal by a mapping circuitry.11. The method of claim 10, further comprising: providing the electricalsignals to electrodes of a cochlear implant.
 12. The method of claim 11,wherein providing the electrical signals to electrodes of the cochlearimplant comprise transmitting the electrical signals wirelessly.
 13. Themethod of claim 9, where receiving an audio signal comprises: receivingan audio signal from a microphone by the bandpass filter.
 14. The methodof claim 9, further comprising: emulating by a phase locking mechanismof the processor a phase relationship between a ganglion cell responseand a basilar membrane vibration period in a healthy cochlea.
 15. Themethod of claim 9, wherein the data indicative of a firing rate ofauditory nerve fibers in a health ear comprises a maximum and a minimuminterval.